Continuous-wave
spatially resolved near infrared spectrometer (NIRS) is used to monitor
changes in oxyhemoglobin and deoxyhemoglobin over a volume of cerebral
tissue. The theoretical underpinnings of this method have been well
described in the literature and involve complex pathways of reflection
and refraction within the cranium. Spatially resolved NIRS can target a
specific area of the brain. To do this subjects are instrumented
with an emitter and adetector
pair of NIRS probes typically on the forehead over the frontal cortex to
monitor absorption of light across the cerebral frontal area. This area
corresponds to the region primarily perfused by the middle cerebral artery. Spacing between optodes needs to be
optimized, and headset sizing and placement adjusted to ensure optimal
signal fidelity strength for each subject during each clinical encounter.
Spatial resolution insures that only infrared signals from the brain were
analyzed and cutaneous signals were excluded. The sampling rate of the
Oxymon was 50Hz, and an integrated digital to analog convertor provided a
reconstructed analog signal. The modified Beer-Lambert Law is used to
calculate micromolar (µmolar) changes in tissue oxyhemoglobin (HbO2)
and deoxyhemoglobin (Hb) across time using optical densities from of near
infrared light at 780 and 850 nm. Often published differential
path-length factors for each subject. Although illuminated cerebral volume may
differ from subject to subject, the assumption is made that the same
volume of the cerebrum is illuminated for a particular subject throughout
a protocol performed on a given day. Changes in total Hb (THb) can
be obtained by adding changes in HbO2 to changes in Hb so
as to index change in blood volume within the illuminated brain volume.
Since only changes in Hb and HBO2 could be measured, the
averages of HbO2 and Hb during quiet rest define the resting
baseline. With appropriate assumptions the technique can be used to
estimate cerebral venous saturation, using methods derived from venous
occlusion plethysmography and by compressing the jugular vein, and
estimates of the absolute total blood volume containing hemoglobin as well
as changes in blood flow entering and leaving the area of the brain that
is illuminated.

The NIRS optodes are here placed on
the right anterior temporal area to approximately illuminate the
area of perfusion of the right middle cerebral artery.

The
figure shows the response of cerebral oxyhemoglobin (HbO2) and total
hemoglobin (THb) to bolus administration of sodium nitroprusside (SNP)
followed by phenyleprhine whcih together comprise the modified Oxford
maneuver used in tests of baroreflex function. These are representative
fits of the NIRS total hemoglobin response (THb in white) to raw THb data
and the fit of oxyhemoglobin response (HbO2 in gray) to raw HbO2
data (light gray line).